Lead and manganese co-activated calcium metasilicate phosphor (CaSiO.sub.3 :Pb,Mn) is a broad-band orange-red emitting phosphor used in fluorescent sign tubing and as a component in multi-phosphor blends for fluorescent lamps. Its preparation and use have been described in U.S. Pat. Nos. 5,472,636, 5,207,948, 2,542,349, 2,542,322, 2,525,028, 2,497,140, 2,474,193, and 2,299,510. The general method of preparation uses silicic acid or silica gel as the source of SiO.sub.2 and dry-blending the SiO.sub.2 with other raw materials (e.g., CaCO.sub.3, PbF.sub.2, and MnCO.sub.3). The raw blend is fired subsequently at times and temperatures sufficient to form crystalline CaSiO.sub.3 :Pb,Mn. The relatively coarse nature of the silica sources, typically 5 to 12 .mu.m, requires that long firing times, 12-20 hours, be used to form the finished CaSiO.sub.3 :Pb,Mn phosphor.
The use of fine silica in the form of a colloidal suspension as a raw material for the synthesis of silicate phosphors is also known (U.S. Pat. Nos. 5,234,625, 5,472,636, 5,611,961). These methods of preparation involve the mixing of a dispersion of the raw materials with the SiO2 suspension at temperatures of 50-90.degree. C., followed by filtering of the resulting precipitate. This step is required for the flocculation and particle growth of the fine SiO.sub.2 so that it can be filtered out of solution. However, the flocculation is dependent not only upon the degree of heating of the suspension, but is also sensitive to the surface charge and solubility of the materials being added to the suspension (carbonates, oxides, etc.), as well as to the intrinsic size of the fine silica being utilized. In the synthesis of CaSiO.sub.3 :Pb,Mn from colloidal silica, a filterable mixture cannot be obtained simply by adding the preferred starting materials to the suspension and then heating. That is, the fine silica does not flocculate, and therefore cannot be mechanically separated from solution. Even in cases where the raw mixture does filter, long times, more than 5 hours, are often required for this step because the filter is blinded by the fine particles. The formation of a homogeneous mixture of the raw materials (including silica) is not optimum under these conditions, because the fine SiO.sub.2 tends to separate from the other materials during the long filtering step. Thus, even longer firing times, 15-30 hours, are required to prepare CaSiO.sub.3 :Pb,Mn phosphor according to these methods. In addition, it is sometimes necessary to utilize multiple firing steps with the addition of reaction aids between steps.
The long filtering and firing times and possible additional steps dictated by the prior art methods increase the manufacturing cost of the CaSiO.sub.3 :Pb,Mn phosphor. Thus, it would be an advantage to have a single step firing method which substantially reduced the filtering and firing times needed to produce the CaSiO.sub.3 :Pb,Mn phosphor.